1. Field of the Invention
The present invention relates to a photovoltaic element for a solar cell, a photo sensor, etc, and a method for producing the same.
2. Related Background Art
Solar cells employing photovoltaic elements are expected to be used as an alternative energy source in place of existing power generation using fossil fuels and hydroelectric power generation, thereby solving the problems of these conventional power generation methods. In particular, various studies have been made on amorphous silicon solar cells, because these cells can be made at relatively low cost and can be produced as elements that have a larger area than solar cells using crystalline silicon. Improvement of the photoelectric conversion efficiency of amorphous silicon solar cells is one of the most important problems to be solved for commercializing the amorphous silicon solar cells. Extensive studies have thus been made for solving the problems described below.
A structure of amorphous silicon solar cell elements is known in which a back electrode, an amorphous silicon semiconductor layer, and incident surface electrode are laminated in this order on a conductive substrate such as a stainless steel plate. The incident surface electrode is made, for example, from transparent conductive oxides.
Furthermore, a collector electrode comprising fine metallic wires is placed on the incident surface electrode mentioned above for collecting the generated electricity. The collector electrode mentioned above is provided on the incident surface; consequently it reduces the effective generating area of the solar cell. The area loss is called the shadow loss. For this reason, the collector electrode is usually made in a fine comb shape. Thus, the collector electrode normally tends to be fine and of a long shape, and appropriate selection of the material and the design of the cross sectional shape thereof are required so as to minimize the electric resistance.
An electrode called a bus bar electrode is formed on the surface of the collector electrode, for collecting the electric current collected by the collector electrode. The bus bar electrode is made from a metal wire that is thicker than the wire of the collector electrode.
Now, the present situation of the research being conducted for minimizing shadow loss and loss by electric resistance and for improving the conversion efficiency of solar cells that are constructed as described above will be explained.
Materials that have small resistivity such as silver (1.62xc3x9710xe2x88x926 xcexa9cm) or copper (1.72xc3x9710xe2x88x926 xcexa9cm) are used for the collector electrode to reduce the shadow loss and electric resistance loss.
Vacuum evaporation, plating, and screen printing methods are used to form the collector electrode.
The vacuum evaporation method has problems such as a slow deposition rate and low throughput, caused by the use of vacuum process, and the necessity of masking to form the linear pattern, which masking results in the loss of metal and deposition on the masked portions. The problem with the screen printing method is the difficulty in forming low resistance electrodes.
For example, the resistivity of the lowest resistance conductive paste is about 4.0xc3x9710xe2x88x925 xcexa9cm, which is one order higher than that of pure bulk silver. The following methods are used to reduce the resistance without areal reduction of the collector electrode using such a material.
(a) Increasing the thickness of the electrode. In this case, the practically usable upper limit of the thickness is 10 xcexcm to 20 xcexcm. When this thickness of electrode is used to form a long, for example, more than 10 cm long collector electrode, it is necessary to make the width of the electrode more than 200 xcexcm in order to keep the electric resistance loss small, and the aspect ratio (ratio of thickness to width) becomes a small value, such as 1:10, and the shadow loss becomes larger. (b) a collector electrode that is made by coating a metal wire with a conductive particle containing polymer is proposed in U.S. Pat. Nos. 4,260,429 and 4,283,591. The cross section of the collector electrode proposed in U.S. Pat. No. 4,260,429 is shown in FIG. 1A. In this figure, reference numeral 101 is a metal wire, and reference numeral 102 is a coating layer made of conductive polymer. This invention has merit in that even a long electrode that is made using the copper wire has a small electric resistance loss, and the shadow loss is also small because the aspect ratio can be a made a small value such as 1:1. The collector electrode proposed in U.S. Pat. No. 4,260,429 can be fixed to the cell by a simple method using conductive adhesive. A method to prevent physical contact between the metallic electrode and a Cu2S layer of the cell is proposed in U.S. Pat. No. 4,283,591; this method provides prevention of the metallic copper deposition.
However, these proposals have the following problems.
(1) In the case of U.S. Pat. No. 4,260,429:
A) The following problems were found as a result of a long term exposure test or by temperature-humidity tests: a short circuit between the upper electrode and the lower electrode is formed at a defective part such as a pin-hole; lower conversion efficiency results from the small shunt resistance, and the yield tends to get worse. Experiments by the present inventors showed that the problem arises from electro-chemical reaction in which the ions from the above mentioned metal wire diffuse through the conductive polymer and reach the above mentioned semiconductor element.
B) The electrode disclosed by U.S. Pat. No. 4,260,429 proposes to obtain good electro-conductivity between the metal wire and the semiconductor element, but a solution of the problem of occurrence of the electrochemical reaction between the metal wire and the semiconductor element is not included.
C) The electrode disclosed by U.S. Pat. No. 4,260,429 has a problem that some portion of the electrode may have not enough bonding force to the semiconductor element. On some occasions, tab portion(s) of the metal material did not have enough bonding force when adhesive connection between the solar cell substrate and metal tab of the collector electrode was required.
D) Not only initial bonding force but long term bonding force between the electrode and the solar cell is required for the solar cell used in an open atmosphere under severe conditions. The solar cells that used electrodes as described above had a problem that a series resistance increase and a conversion efficiency decrease caused by deterioration of the bonding force occurred during the accelerated temperature-humidity test and heat resistance test.
E) Some problems of electrode peeling were observed with solar cells caused by poor initial bonding force between the solar cell substrate and metal tab, and also by degradation of the bonding force between the cell element, metal wire, and the coating layer, affected by the humidity and temperature.
F) The solar cell was readily affected by the humidity because a tight covering layer was not formed as a covering film.
G) It is desirable that the covered wire electrode can be separately manufactured and be storable. However, in the case where a thermosetting resin was used the above mentioned electrode had a problem that it was difficult to obtain sufficient bonding force when it was formed on the solar cell because the cure rate of the polymer after the drying step was difficult to control. Furthermore, there was no means for the selection of the curing agent for the thermosetting resin and a relatively long curing time was required.
H) When only thermoplastic resin was used, deformation of the electrode occurred due to thermo-hysteresis during the lamination process after the formation of the electrode, and the following problems were observed: line width change, partial peeling, and position shift of the electrode.
I) For solar cells that are used while open to the atmosphere, it is required that there be no change in the bonding force between the electrode and the solar cell element even if it is used for a long term under severe conditions. The solar cells that used the above mentioned electrode had a problem that a series resistance increase and a conversion efficiency decrease was caused by the deterioration of the bonding force during long term open air exposure test or temperature-humidity tests as accelerated tests.
(2) In the case of U.S. Pat. No. 4,283,591.
A) Although the idea of preventing physical contact between the metallic electrode and the semiconductor layer was disclosed, a solution of the problem, in which the metal ion diffuses slowly through the conductive polymer and induces trouble, was not proposed.
B) The electrode proposed by this invention has a possibility that the metal wire may contact the solar cell substrate as a result of breakage of the above mentioned covering layer during the thermal crimping process. A specific counter measure for this problem is not proposed.
C) The proposal has some limit to the electrode formation process because the procedure does not contain a drying step and the coated wire cannot be stored.
(3) In the cases of U.S. Pat. Nos. 4,260,429 and 4,283,591:
A) Either proposal has the problem that it is difficult to obtain a coating layer of uniform thickness and stable good electric conductivity.
B) Short circuits between the upper electrode and the lower electrode are formed when the coating layer has pin holes that induce a large enough leakage current. As a result the shunt resistance decreased, lower conversion efficiency results, and the yield decreases.
C) The electrode proposed by the invention has a possibility that the metal wire may contact the solar cell substrate, and when it is used outdoors the effect of migration and shunt closure was not studied.
(4) In the case of U.S. Pat. No. 5,084,104.
A) Short circuits between the upper electrode and the lower electrode are formed when an amorphous silicon solar cell that has defective parts such as pin holes was used, and lower conversion efficiency results from the small shunt resistance, and the yield tends to get worse.
B) Series resistance of the electrode that is covered by the conductive adhesive increases due to thermo-hysteresis because of the dissolution or softening of the electrode caused by the penetration of the paint solvent.
C) The series resistance of the photovoltaic element increases and the conversion efficiency decreases when it is subjected to accelerated open air exposure testing or temperature-humidity testing.
An object of the present invention is to provide a collector electrode that has excellent storability, adhesion, and resistance to leakage due to humidity.
Another object of the present invention is to provide a photovoltaic element that has high initial characteristics and long term reliability, using the above mentioned collector electrode, avoids short circuits between the upper electrode and the lower electrode, avoids penetration of the paint solvent to the electrode covered by the conductive adhesive, and prevents increase of series resistance.
Still another object of this invention is to provide a manufacturing process that can make the photovoltaic element in a high yield and in stable operation.
According to a first aspect of the present invention, a photovoltaic element is provided of a structure in which an electrode coated with a conductive adhesive is placed on a photoactive semiconductor layer via a conductive adhesive, wherein the conductive adhesive is composed of at least two layers; and the softening point of the conductive adhesive composing the layer nearer to the electrode is higher than the highest temperature encountered in the manufacture of the photovoltaic element.
According to a second aspect of the present invention, a collector electrode is provided in which the metal wire does not contact the photovoltaic element directly because a coating layer comprising a conductive resin is provided on the metal wire, wherein the ions of the metal do not diffuse into the semiconductor layer of the photovoltaic element.
According to a third aspect of the present invention, a collector electrode in which a metal wire comprising a coating layer consisting of a conductive adhesive is adhered to photovoltaic elements via said coating layer, wherein metal ions of said metal wire do not diffuse into the semiconductor layer of said photovoltaic elements.
According to a fourth aspect of the present invention, a photovoltaic element which comprises a semiconductor layer consisting of at least one pin junction or pn junction and a collector electrode is provided on the light incident side of the semiconductor layer, wherein the collector electrode comprises the collector electrode of the second or third aspects.
According to the fifth aspect of the present invention, a method is provided for manufacturing a photovoltaic element of a structure having a collector electrode on the light incident side, wherein the collector electrode of the second, third or fourth aspects is adhered to the light incident face of the photovoltaic element by means of heat or pressure, or both.